Aromatic hydrocarbon recovery process



Sept. 9, 1969 R. R. DE GRAFF ET AL 3,466,345

AROMATIC HYDROCARBON RECOVERY 'PROCESS Filed May 29, 1967 Ric/:afd R. Deraff Mart/'fp W. Perga Arom'af/c Exlrac/on United States Patent Office 3,466,345 Patented Sept. 9, 1969 U.S. Cl. 260-674 5 Claims ABSTRACT OF THE DISCLOSURE Method for solvent extracting aromatic hydrocarbons from suitable feedstocks such as catalytic reformate and for regenerating the solvent to a quality suitable for reuse in the solvent extraction zone. The preferred solvent comprises sulfolane and the typical aromatic hydrocarbons to be recovered include fbenzene, toluene, and xylene.

BACKGROUND OF THE INVENTION The present invention relates to a process for recovering aromatic hydrocarbons from a feed stream containing aromatic and non-aromatic hydrocarbons. It specifically relates to a method for the recovery of aromatic hydrocarbons and solvent regeneration techniques. It particularly relates to an integrated method for recovering aromatic hydrocarbons and regenerating solvent suitable for reuse in the aromaticv solvent extraction step.

It is known in the art that a conventional process for the recovery of high purity aromatic hydrocarbons of, say, nitration grade from various feedstocks including catalytic reformates is liquid-liquid extraction utilizing a solvent such as diethylene Iglycol or sulfolane, each of which has high selectivity for the desired aromatic hydrocarbon components contained in the feedstock. Typically, in the practice of such prior art process a hydrocarbon feed mixture is contacted in an extraction zone withv an aqueous solvent composition which selectively dissolves the aromatic component of the hydrocarbon feedstock thereby forming a raffinate phase comprising one or more non-aromatic hydrocarbons and an extract phase containing dissolved aromatic components. T he extract phase is then separately distilled yielding an overhead distillate containing only a portion of the extracted aromatic component, a sidecut fraction comprising aromatic hydrocarbons and a bottoms fraction comprising lean solvent suita'ble for reuse in the extraction zone. Frequently to prevent losses of the solvent, the raffinate phase is washed with Water in a washing zone in order to remove solvent from the raffinate phase.

Also, not infrequently, the extract phase is subjected to extractive distillation in order to remove a contaminating quantity of non-aromatic hydrocarbons from the extract phase. This extractive distillation operation is normally performed in order to make possible the recovery of nitration grade aromatic hydrocarbons such as benzene and toluene. Therefore, a typical prior art process for the recovery of aromatic hydrocarbons encompasses a solvent extraction step, an extractive distillation step, and a final distillation step for recovery of high purity aromatic hydrocarbons from the solvent phase. Another prior art step, more fully discussed hereinbelow, is a solvent regeneration step which has as its function the recovery of regenerated solvent suitable for reuse in the extraction zone.

The solvents which are applicable to the practice of the present invention and to the aromatics extraction process, generally, are known to be thermally unstable. The instability is not pronouced, however, and only becomes evident upon prolonged recycling of the solvent whereupon the accumulation of the decomposition products becomes evident. Generally, the rate of decomposition increases with increasing operating temperatures. Thus, it has ybeen found that the rate of decomposition, for example, of sulfolane in an inert atmosphere is 0.002% per hour at 200 yC., 0.01% per hour at 220 C., and 0.02% per hour at 230 C. Similar thermal effects are observed with other satisfactory solvents and it is therefore desirable to keep temperature levels as low as possible. Accordingly, it is the practice, for example when using the sulfolane solvent system, to set a maximum process temperature of about 350 F. while in the diethylene glycol solvent system it is the practice to set a maximum process temperature of about 380 F. Consequently, the prior art defines such processing temperatures as being the point of thermal instability; although, it is known that there is some decomposition occurring below those temperature levels, and in some instances, temperatures above these temperature limits may be utilized for short periods of time. Similar points of thermal instability may be readily ascertained for other solvent systems.

It is known that the solvent decomposition results in the production of acidic organic deterioration products as well as polymerization products of a resinous character. It is further believed that the decomposition is accelerated by the presence of oxygen. The exact nature of the final decomposition products is not fully known, but where sulfolane is the solvent, the decomposition initially produces sulfur dioxide, sulfur trioxide, and olefins.

The presence of organic acids within the aqueous Solvent and of sulfurous gases within an aqueous sulfolane solvent tends to cause accelerated corrosion of the steel materials used in the construction of the process unit. Therefore, it is the usual prior art practice to add organic amine compounds to the solvent composition as corrosion inhibitors. Suitable organic amines for use in the solvent composition may be selected from the aliphatic, aromatic, naphthenic, and hetrocyclic amines, generally, as well as the alkanolamines containing one or more amine groups and/or hydroxy groups per molecule. The amine may also be a primary, secondary, or tertiary amine, but the preferred amine utilized as a corrosion inhibitor in the sulfolane solvent system is an alkanolamine, and more particularly, monoethanolamine. Because of the basic characteristics of these amine inhibitors, these materials react with the acidic solvent decomposition products to produce amine salts and amides at the temperature conditions utilized in the aromatic extraction process.

With continued recycling or circulation of the solvent composition within the processing equipment the solvent tends to accumulate in undesirable quantities resinous polymeric decomposition products, amine salts of the acidic solvent decomposition products, amides of the acidic solvent decomposition products and other relatively non-volative constituents from other sources. All of these are characterized individually and collectively as being relatively non-volatile contaminants `and the accumulation thereof results in the eventual precipitation of tarry insoluble deposits on the interior surfaces of the processing equipment resulting in reduced heat transfer etliciency due to fouling of heat exchangers and resulting in reduced separation efficiencies due to fouling of extractor decks and fractionating column trays.

In an effort to overcome the problems associated with solvent deterioration, the prior art practice has been to withdraw from the lean solvent stream a drag stream of lean solvent for solvent regeneration and recovery of clean lean solvent suitable for reuse in the solvent extraction step. The withdrawal rate is normally suicient to providethat the entire solvent inventory of the aromatics extraction process is passed through the solvent regeneration system maintained under vacuum once every 5 to 10 days. In this manner, the relatively non-volatile contaminants never accumulate to a suiciently high concentration to cause deposition of tarry insoluble sludge which is otherwise encountered in the solvent circulating system. However, operating under vacum presents problems of air leakage into the system which tend to aggravate solvent decomposition.

SUMMARY `OIF THE INVENTION It is therefore an object of this invention to provide a method for the recovery of aromatic hydrocarbons from the extra-ct phase of a solvent extraction operation while maintaining solvent cleanliness at a desirable high level.

It is another object of this invention to provide a method for fractionating a sulfolane solvent having dissolved therein aromatic and non-aromatic hydrocarbons in a facile and economical manner.

Thus, according to one embodiment of this invention, there is provided a method for recovering aromatic hydrocarbons from rich solvent having aromatic hydrocarbons dissolved therein which comprises the steps of: (a) introducing said rich solvent into a rst separation zone maintained under distillation conditions including the presence of hereinafter specified stripping medium; (b) withdrawing from said first zone a distillate fraction comprising aromatic hydrocarbons and a bottoms fraction comprising lean solvent having relatively non-'volatile contaminants therein; (c) passing at least a portion of said bottoms fraction into a second separation zone maintained under separation conditions; (d) introducing stripping medium into the lower portion of said second zone in an amount sufficient to aid in removing solvent from said contaminants; (e) withdrawing from said second zone a vapor fraction comprising regenerated solvent substantially free of contaminants and containing said stripping medium; (f) passing vapor fraction including stripping medium from the second zone into the lower portion of the first zone; and, (g) recovering aromatic hydrocarbons in high concentration from said first zone.

It is noted from the hereinabove brief description of the present invention, relative to the prior art, that significant economies of operation are achieved by passing the entire overhead product from the solvent regenerator back into the aromatic hydrocarbon recovery column in such a manner that the stripping medium utilized in the regeneration column is also utilized as stripping medium in the aromatic recovery column. The condensing system for the solvent regenerator, liquid recovery system, and vacuum system have all been eliminated at substantially no increase in duty for these items on the recovery column. Heat input to the regenerator reboiler is utilized in the recovery column resulting in additional savings in utilities and operation at higher pressure reduces air leakage problems. In many cases it has been found that the stripping medium added to the solvent regenerator is the only stripping medium necessary for the operation of both of these columns. On the other hand, an additional amount of stripping steam may be necessary in the aromatic recovery column for those instances when relatively small amounts of lean solvent are being regenerated. It is desirable that the quantity of steam to the solvent regenerator be not less than that amount required in the recovery because: (l) for a fixed solvent regeneration rate, more steam requires lower temperature and results in less degradation; (2) for a fixed maximum temperature, more steam will allow reduction of quantity of solvent in the regenerator bottoms.

The hydrocarbon feed mixture which may be separated by the improved method of the present invention comprises broadly many different aromaticnonaromatic mixtures. Typically, feedstocks applicable to the solvent extraction technique include hydrocarbon distillate fractions (usually boiling within or near the gasoline boiling range) of natural gasoline or straight-run petroleum distillates, and especially comprises reformed naphthas which are rich in aromatic hydrocarbons and which are particularly valuable as source of such mononucler aromatic hydrocarbons as benzene, toluene, `and xylene. Thus, the desired aromatic hydrocarbons may comprise benzene, toluene, benzene and toluene, toluene and xylene; and benzene, toluene, and xylene; Cg-iaromatic hydrocarbons, etc. In each case, however, it is understood that the feedstock to the aromatic hydrocarbon recovery column contains a relatively high percentage or aromatic hydrocarbons dissolved in a suitable solvent which is contaminated with relatively non-volatile material.

Solvent compositions which may be utilized in the practice of the present invention are those selected from the classes which have high selectivity for aromatic hydrocarbons. These aromatic selective solvents generally contain one or more organic compounds containing in their molecule at least one polar group such as a hydroxyl, amino, cyano, carboxyl, or nitro radical. 'In order to be effective, the organic compounds of the solvent composition having the polar radical must have a boiling point substantially greater than the boiling point of water, which preferably included in the solvent composition for enhancing its selectivity and in general must also have a boiling point substantially greater than the end boiling point of the aromatic component to be extracted from the hydrocarbon feed mixture.

Organic compounds suitable for use as part of the solvent composition preferably are selected from the group of those organic containing compounds which include the aliphatic and cyclic alcohols, cyclic monomeric sulfones, the glycols and glycol ethers, as well as the glycol esters and glycol ether esters. The monoand poly-alkylene glycols in which the alkylene group contains from 2 to 3 carbon atoms such as ethylene glycol, diethylene glycol, triethylene glycol and tetraethylene glycol, propylene glycol, diproplyene glycol, and tripropylene glycol, as well as the methyl, ethyl, propyl, and butyl ethers of the glycol hydroxyl groups, and the acetic acid esters thereof, constitute a particularly preferred class of organic solvents useful in admixture with water as the solvent composition for use in the present invention. An illustrative glycol comprises triethylene glycol.

Additionally, excellent results may be obtained utilizing the cyclic monomeric sulfone, such as tetrahydrotriophene-l,ldioxide. Still further, an organic compound particularly useful in the practice of this invention is a sulfolane which may be made by condensing a conjugated diolen with sulfur dioxide and then subjecting the resulting product to hydrogenation, alkylation, hydration and/ or other substitution or addition reactions. Typically, organic compounds belonging to the sulfolane class are 2- sulfolene, 2-methyl sulfolane, 2,4-dimethylsulfolane, 2,4- dimethyl-4-sulfolane, methyl-3-sulfonyl ether, ethyl-3- sulfonyl sulfide, and others.

The apparatus embodied in the practice of the invention may be any conventional or convenient type known to those skilled in the art. Also the operation conditions suitable for the practice of this invention are conventional. Generally, the amount of solvent composition utilized in admixture with an appropriate feedstock should be at least suicient to dissolve the constituents to be extracted. It may be desirable to use a considerable excess over the theoretical amount of solvent composition necessary, especially when maximum purity and maximum recovery of aromatic Ihydrocarbons is required. Usually, in the extraction step, the solvent composition to feed ratios will range from about 1:1 to about 20:1 by volume, preferably, from about 5:1 to about 15:1 by volume. A summary of the conditions necessary for the practice of the sulfolane type of solvent operation may be found in Petroleum Rener, Volume 38, No. 9, Sept. 1959, pages 18S-192, the entire disclosure of which is incorporated herein by reference.

The solvent extraction step, as previously mentioned, is well known and may utilize apparatus of any type suitable for effecting counter-current contact between two liquid phases at least partially, but not wholly miscible with each other wherein the relatively more dense solvent may be brought into intimate contact with the relatively less dense 'hydrocarbon phase. Thus, the extraction zone which produces the solvent extract which is ultimately charged as feedstock to the aromatic hydrocarbon recovery column may comprise a packed column or may contain a series of horizontal plates through which the liquid solvent flows in dispersal form and in counter-current relationship to the ascending stream.

The solvent regnerating system comprises a distillation column which is generally operated under vacuum in order to minimize the vaporization temperature of the thermally unstable solvent. Vaporization frequently is accomplished by provision of a reboiler heat exchanger and the solvent vapor containing inhibitor and water is moved overhead, and in the practice of this invention passed into the lower portion of the aromatic hydrocarbon recovery column. A major portion of the lean solvent stream from the bottom of the aromatic recovery column is returned, generally, to the solvent extraction zone. Not infrequently, a portion of this recycle stream is diverted into the extractive distillation column as previously mentioned so that an excess of solvent is present in that column for enhancing the ease with which contaminating non-aromatic hydrocarbons are removed from the solvent containing the aromatic hydrocarbons.

The invention may be more fully understood with reference to the accompanying drawing which is a schematic reperesentation of apparatus for practicing broadly one embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWING lReferring now to the drawing a suitable hydrocarbon feedstock such as catalytic reformate is contacted with an appropriate quantity of solvent from line 11 and passed via line into aromatic extraction zone 12. Suitable conditions are maintained in extraction zone 12 to produce a raffinate phase containing primarily non-aromatic hydrocarbons which are removed from zone 12 via line 13 and to produce an extract phase comprising solvent Ihaving aromatic hydrocarbons dissolved therein which are removed from zone 12 via line 14.

The extract phase in line 14 has primarily aromatic hydrocarbons dissolved in a solvent such as sulfolane and due to inherent ineiciencies of any commercial solvent extraction step, this extract phase is also contaminated with a small amount of non-aromatic hydrocarbons. lIf required by the composition of the extract, the extract feed in line 14 is mixed with added solvent (not shown) and the combined extract plus added solvent is passed into extractive distillation column 15 which is maintained under distillation conditions.

Operating conditions in distillation column 15 are conventional in that sufficient heat Imust be added to the column in order for separation to take place between the non-aromatic hydrocarbons and aromatic hydrocarbons dissolved in the solvent. Typical operating conditions for distillation column 15, suitable when using a sulfolane type solvent', include a pressure from 90r mm. Hg to l5 p.s.i.g. limited by decomposition temperature of solvent, an overhead'temperature from 140 F. to 330 F., and a bottoms temperature from 170 F. to 350 F., preferably no higher than 350 F. Operating under these conditions, a non-aromatic hydrocarbon-containing stream is withdrawn from column 15 via line 16 and frequently is recycled as reflux to the extraction zone. A lower stream comprising solvent and aromatic hydrocarbons is withdrawn, preferably, from the bottom of column 15 via line 17 and passed into aromatic hydrocarbon recovery column 18 generally at an upper location therein. Suitable distillation conditions are maintained in column 18 in order to produce a distillate fraction comprising aromatic hydrocarbons which are withdrawn from the process via line 19. The bottoms of column 18 comprise contaminated lean solvent. Typically, the contaminated lean solvent stream comprises sulfolane and contains from about 0.5 to about 1.0 percent by weight water, traces of amine corrosion inhibitor, traces of amine salts of acidic sulfolane decomposition product, traces of amides of acidic sulfolane decomposition products, traces of resinous polymeric sulfolane decomposition products, and traces of relatively non-volatile contaminants. This bottoms stream is removed from colum 18 via line 20 and passed at least in part into solvent regeneration column 21.

Operating conditions in solvent regeneration column 21 include a pressure from 200 mm. Hg to 800 mm. Hg, typically, about 500 mm. Hg, and a temperature from 250"` F. to 350 F., typically about 300 F. suicient to produce an overhead vapor fraction comprising sulfolane solvent and water in the form of steam. This overhead product, as a vapor, is withdrawn from solvent regeneration zone 21 via line 23 and passed directly into distillation column 18 at a lower section thereof. Suicient stripping medium such as steam is added to solvent regeneration column 21 via line 22. Generally, the amount of stripping medium added is sufficient to aid in the removal of solvent from its relatively non-volatile contaminants. As a measure of the amount of stripping medium necessary when steam is used, an amount of 1 part per part of contaminated solvent by weight to 3 parts per part by weight is generally satisfactory. A residue stream comprising the relatively nonvolatile contaminants is removed from column 21 via line 24 and passed into disposal means known to those skilled in the art.

A major portion of the contaminated lean solvent stream in line 20 is diverted by line 11 and passed with the incoming feed to be extracted, as previously mentioned. By means not shown, a portion of the material in line 11 may be diverted into distillation column 15, also as previously mentioned.

Returning now to regenerator column 21, the bottom of column 21 comprises, preferably, a liquid reservoi1 containing heat exchanger means whereby the non-volatile lcontaminants of the solvent may be accumulated. In the practice of this invention it may be desirable to only remove this residue via line 24 on a periodical basis; however, it is within the concept of the present invention to remove this material continually if practical and to remove the material continually through the aid of a cutlback oil which may be added to column 21 by means not shown.

PREFERRED EMBODIMENT Therefore, from the teachings presented herein, the preferred embodiment of this invention is an improvement to a process for regenerating sulfolane solvent contaminated with relatively non-volatile constituents, said contaminated solvent being the bottoms product from an aromatic hydrocarbon distillation recovery column, which comprises introducing contaminated solvent into a regeneration zone maintained under distillation conditions; injecting stripping steam into the bottom of the regeneration zone; withdrawing from the zone a vapor fraction comprising regenerated solvent substantially free of contaminants together with said stripping steam; and, passing vapor fraction including stripping steam into the lower The invention claimed:

1. Method for recovering aromatic hydrocarbons from rich solvent having aromatic hydrocarbons dissolved therein which comprises the steps of:

(a) introducing said rich solvent into a first separation zone maintained under distillation conditions including the presence of hereinafter specified stripping medium;

(b) withdrawing from said rst zone a distillate fraction comprising aromatic hydrocarbons and a bottoms fraction comprising lean solvent having relatively non-volatile contaminants therein;

(c) passing at least a portion of said bottoms fraction into a second separation zone maintained under separation conditions;

(d) introducing stripping medium into the lower portion of said second zone in an amount sufficient `to aid in removing solvent from said contaminants;

(e) withdrawing from said second zone a vapor fraction comprising regenerated solvent substantially free of contaminants and containing said stripping medium; .l

(f) passing vapor fraction including stripping medium from the second zone into the lower portion of. the rst zone; and, o y v (g) recovering aromatic hydrocarbons in high concentration from said rst zone. g

2. Method according to claim 1 wherein said solvent comprises sulfolane.

3. Method according to claim 1 wherein saidrstripping medium comprises Steam.

4. In a process for regenerating sulfolane solvent contaminated with relatively non-volatile constituents, said contaminated solvent being the bottoms product from an 8 aromatic hydrocarbon distillation recovery column, the improvement which comprises introducing contaminated solvent into a regeneration zone maintained under distillation conditions; injecting stripping steam into the bottom of the regeneration zone; withdrawing from the zone a vapor fraction comprising regenerated solvent substantially free of contaminants together with said stripping steam; and, passing vapor fraction including stripping steam into the lower section of said aromatic recovery column.

5. Improvement according to claim 4 wherein said aromatic hydrocarbon comprises benzene.

References Cited UNITED STATES PATENTS 3,146,190 8/1964 Papadapoulos et al. 28-325 XR 3,222,416 12/1965 Evans et al. 260-674 3,338,823 8/1967 Voetter 208-321 XR 3,361,664 1/1967 Broughton et al. 260-674 XR DELBERT E. GANTZ, Primary Examiner C. R.,DAVIS, Assistant Examiner Us. c1. XR. 20s-321, 325 

